Feed-forward observer-based control for estimating cylinder air charge

ABSTRACT

A system and method for controlling an internal combustion engine to improve air/fuel ratio control using a feed-forward observer-based control strategy include estimating current and future airflow actuator positions, determining corresponding mass airflow values, and determining a future in-cylinder air charge based on a difference between the current and future mass airflow values. In one embodiment, the difference between current and future mass airflow estimates is used as a feed-forward term and combined with a sensed or measured value generated by a mass airflow sensor prior to being processed using a manifold filling model to predict the future cylinder air charge. In another embodiment, the current and future mass airflow estimates are processed using the manifold filling model with the results used to generate a future delta cylinder air charge that is subsequently used as a feed-forward term and combined with a measured air charge corresponding to a measured mass airflow processed by the manifold filling model.

This application is a divisional of application Ser. No. 09/570,784,filed on May 13, 2000, now U.S. Pat. No. 6,460,409 entitled“Feed-Forward Observer-Based Control For Estimating Cylinder AirCharge”, assigned to the assignee of the parent application, andincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to systems and methods for cylinder aircharge estimation used in controlling an internal combustion engine.

BACKGROUND ART

Precise air/fuel ratio control is an important factor in reducing feedgas emissions, increasing fuel economy, and improving driveability.Current internal combustion engine designs use various temperature,pressure, and flow sensors in an attempt to precisely control the amountof air and fuel, and thus the air/fuel ratio, for each cylinder firingevent. However, due to various sensor limitations such as response timeand being located away from the combustion chamber of the cylinder, itis difficult to precisely measure and coordinate or synchronize the airand fuel quantities which are actually combusted in the cylinder.Acceptable control strategies have been developed to compensate forvarious sensor limitations under steady-state operating conditions.Effort is now being focused on improving these strategies to providemore accurate air/fuel ratio control during transient as well assteady-state operating conditions.

Electronically controlled throttle valve actuators have been used toimprove transient air/fuel ratio control by providing increased controlauthority over airflow. By eliminating the mechanical linkage between anaccelerator pedal and the throttle valve, the engine controller cancontrol throttle valve position to deliver the proper airflow forcurrent driver demand and operating conditions.

Airflow is typically measured using a mass airflow (MAF) sensorpositioned upstream of the throttle valve. Intake air travels past theMAF sensor, through the throttle valve and into the intake manifoldwhere it is distributed to a bank of cylinders. Intake air enters acylinder upon the opening of one or more intake valves. Fuel may bemixed with the intake air prior to entering the cylinder or within thecylinder for direct injection applications. The response characteristicsof current MAF sensors coupled with the delay time associated withthrottle valve positioning, transit time of the air mass between the MAFsensor and the cylinder, and response time of the fuel injector, make itdifficult to accurately determine the precise quantity of air and fuelin the cylinder.

Various prior art approaches have attempted to improve air/fuel ratiocontrol and compensate for one or more of the above factors. Forexample, one approach attempts to synchronize throttle valve positioningcommands and fuel injection commands in the crank-angle domain so thatthrottle valve movement is prohibited after air flow measurement.Another approach delays throttle valve movement to allow time for thefuel system to react. One strategy which provides a future estimate ofcylinder air charge linearly extrapolates a current airflow measurementfor a future fuel injection event. However, this method assumes aircharge changes at a constant rate and does not compensate for airflowsensor filtering effects which lead to an attenuated and delayedresponse.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve air/fuel ratiocontrol using feed-forward observer-based controls to provide anestimate for future cylinder air charge during a future fuel injectionevent.

In carrying out the above object and other objects, features, andadvantages of the present invention, a system and method for controllingan internal combustion engine having an electronically controlledairflow actuator, such as a throttle valve or intake/exhaust valves,include predicting current and future positions of the airflow actuatorusing an actuator model with the future position preferablycorresponding to a subsequent injection of fuel into the cylinder,generating a delta mass air flow prediction based on current and futureair flow estimates, and estimating air charge in the cylinder for thesubsequent injection of fuel based on the delta mass air flowprediction. In one embodiment, the delta mass air flow prediction isused as a feed-forward term which is added to the current mass airflowsensor reading with the result processed by an intake manifold fillingmodel to provide a future estimate of in-cylinder air charge. In anotherembodiment, the current and future mass airflow estimates are processedby an intake manifold filling model to produce corresponding cylinderair charge estimates. The difference of the estimates is then used as afeed-forward term that is combined with the air charge calculated usingthe intake manifold filling model with the sensed mass airflow as aninput.

The present invention includes a number of advantages relative to priorart control strategies. For example, improved air charge estimationusing the present invention allows more precise air/fuel ratio control.A priori knowledge of airflow control actuator position providesadvanced notice via system modeling of how the cylinder air charge willbe effected and allows improved air/fuel ratio control, especiallyduring transient conditions. This may lead to reduced feedgas emissionsand a corresponding reduction in the necessary size of the catalyst. Afaster control system response without inducing instability or noiseusing the present invention may also result in improved driveability.

The above advantages and other advantages, objects, and features of thepresent invention, will be readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one embodiment of an enginecontrol system using cylinder air charge estimation with feed-forwardobserver-based control according to the present invention;

FIG. 2 is a simplified block diagram illustrating operation of a systemor method for controlling an engine according to the present invention;

FIG. 3 is a block diagram illustrating a general dynamic model for aclosed-loop electronic throttle control system for use in determining anestimated future cylinder air charge according to the present invention;

FIG. 4 is a simplified representation of a recursive airflow actuatorposition model for estimating current and first and second futureactuator positions according to the present invention;

FIG. 5 is a more detailed representation of a model for estimatingfuture airflow actuator position according to the present invention;

FIG. 6 is a block diagram illustrating estimation of intake airflow andmanifold filling based on estimated throttle plate angle according toone embodiment of the present invention;

FIG. 7 is a block diagram illustrating a control system implementingfeed-forward observer-based control according to one embodiment of thepresent invention;

FIG. 8 is a block diagram illustrating a control system implementingfeed-forward observer-based control according to another embodiment ofthe present invention; and

FIG. 9 is a flowchart illustrating operation of one embodiment of asystem or method for feed-forward observer-based air charge estimationaccording to the present invention.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

A block diagram illustrating one embodiment of an engine control systemfor an internal combustion engine according to the present invention isshown in FIG. 1. While a direct injection application is depicted inFIG. 1, the present invention is equally applicable to conventional portor throttle body injection systems as well. Similarly, while the presentinvention is described primarily with reference to an electronicallycontrolled throttle to provide airflow control, the present inventionmay also be applied to various other types of airflow actuators such ascylinder intake/exhaust valves used in variable cam timing and variablevalve timing applications with appropriate adjustments to the variousmodels.

System 10 is preferably an internal combustion engine having a pluralityof cylinders, represented by cylinder 12, having correspondingcombustion chambers 14. As one of ordinary skill in the art willappreciate, system 10 includes various sensors and actuators to effectcontrol of the engine. One or more sensors or actuators may be providedfor each cylinder 12, or a single sensor or actuator may be provided forthe engine. For example, each cylinder 12 may include four actuatorswhich operate the intake valves 16 and exhaust valves 18, while onlyincluding a single engine coolant temperature sensor 20.

In one embodiment, the present invention includes a mechanical variablecam timing device of conventional design used to alter the timing ofintake valves 16 and/or exhaust valves 18 to provide airflow control. Inan alternative embodiment, intake valves 16 and/or exhaust valves 18 arecontrolled by variable valve timing actuators, such as electromagneticactuators, as known in the art. One preferred embodiment of the presentinvention uses an electronically controlled throttle for airflow controlas described in detail below.

System 10 preferably includes a controller 22 having a microprocessor 24in communication with various computer-readable storage media. Thecomputer readable storage media preferably include a read-only memory(ROM) 26, a random-access memory (RAM) 28, and a keep-alive memory (KAM)30. The computer-readable storage media may be implemented using any ofa number of known memory devices such as PROMs, EPROMs, EEPROMs, flashmemory, or any other electric, magnetic, optical, or combination memorydevice capable of storing data, some of which represents executableinstructions, used by microprocessor 24 in controlling the engine.Microprocessor 24 communicates with the various sensors and actuatorsvia an input/output (I/O) interface 32.

In operation, air passes through intake 34 where-it may be distributedto the plurality of cylinders via an intake manifold, indicatedgenerally by reference numeral 36. System 10 preferably includes a massairflow sensor 38 which provides a corresponding signal (MAF) tocontroller 22 indicative of the mass airflow. In preferred embodimentsof the present invention, a throttle valve 40 is used to modulate theairflow through intake 34 during certain operating modes. Throttle valve40 is preferably electronically controlled by an appropriate actuator 42based on a corresponding throttle position signal generated bycontroller 22. A throttle position sensor 44 provides a feedback signal(TP) indicative of the actual position of throttle valve 40 tocontroller 22 to implement closed loop control of throttle valve 40.

As will be appreciated by those of ordinary skill in the art, thepresent invention may also be used in unthrottled or throttlelessengines where airflow may be controlled using appropriate valve timing.Whether or not the engine includes a physical throttle, such as throttlevalve 40, the engine may be operated in various unthrottled modes. Suchoperation reduces pumping losses and increases engine efficiency whichmay result in improved fuel economy. Throttleless engines may includethose having variable valve timing (VVT) where intake and exhaust valvesare. controlled electronically using electromagnetic actuators ratherthan a conventional cam arrangement. Likewise, engines having variablecam timing mechanisms may be operated at wide open throttle to reducepumping losses with air flow control provided by modifying the camtiming. The present invention is also applicable to engineconfigurations with conventional valve timing mechanisms which may alsooperate at wide open throttle in various modes depending upon thecurrent driver demand and engine operating conditions.

As illustrated in FIG. 1, a manifold absolute pressure sensor 46 may beused to provide a signal (MAP) indicative of the manifold pressure tocontroller 22. Air passing through intake manifold 36 enters combustionchamber 14 through appropriate control of one or more intake valves 16.As described above, intake valves 16 and exhaust valves 18 may becontrolled directly or indirectly by controller 22 for variable valvetiming or variable cam timing applications, respectively. Alternatively,intake valves 16 and exhaust valves 18 may be controlled using aconventional camshaft arrangement. A fuel injector 48 injects anappropriate quantity of fuel in one or more injection events for thecurrent operating mode based on a signal (FPW), generated by controller22 and processed by driver 50.

As illustrated in FIG. 1, fuel injector 48 injects an appropriatequantity of fuel in one or more injections directly or indirectly intocombustion chamber 14. Control of the fuel injection events is generallybased on the position of piston 52 within cylinder 12. Positioninformation is acquired by an appropriate sensor 54 which provides aposition signal (PIP) indicative of rotational position of crankshaft56.

According to the present invention, the air/fuel ratio may be moreprecisely controlled by providing an estimate of cylinder air charge fora future injection event. Once an appropriate air/fuel ratio isdetermined based on a desired engine torque and current operatingconditions, an appropriate quantity of fuel is determined based on theestimated cylinder air charge to more accurately control the air/fuelratio. Preferably, the cylinder air charge and fuel are determined fortwo PIP events ahead of the current event. Because the PIP events arebased on crank angle, timing between events will vary based on therotational speed (RPM) of the engine. Preferably, one or more airflowactuators are controlled to synchronize the predicted or estimatedcylinder air charge with the scheduled fuel injection event. Inthrottled applications, air flow may be controlled using the throttlevalve in combination with control of valve timing for intake and/orexhaust valves.

The desired fuel flow is achieved during by appropriate signalsgenerated by controller 22 for fuel injectors 48 to inject anappropriate quantity of fuel in one or more injections directly orindirectly into each combustion chamber 14. Depending upon theparticular application, fuel quantity may also be determined or adjustedto account for fuel film or wall wetting which ultimately affects theamount of fuel actually delivered to the cylinder. At the appropriatetime during the combustion cycle, controller 22 generates a spark signal(SA) which is processed by ignition system 58 to control spark plug 60and initiate combustion within chamber 14. Preferably, spark ismaintained at MBT, i.e., the timing that produces maximum torque for agiven amount of air and fuel, whenever possible because these conditionsgenerally result in better fuel economy.

Controller 22 (or a conventional camshaft arrangement) controls one ormore exhaust valves 18 to exhaust the combusted air/fuel mixture throughan exhaust manifold. An exhaust gas oxygen sensor 62 provides a signal(EGO) indicative of the oxygen content of the exhaust gases tocontroller 22. This signal may be used to adjust the desired air/fuelratio, or control the operating mode of one or more cylinders. Theexhaust gas is passed through the exhaust manifold and through acatalytic converter 64 and in some applications a NO_(x) trap 66 beforebeing exhausted to atmosphere.

FIG. 2 provides a simplified block diagram illustrating operation of asystem or method for future in cylinder air charge estimation accordingto the present invention. A dynamic model 80 of a closed-loop airflowactuator system is used recursively to generate estimates for currentand future actuator positions. In this example, model 80 processes acurrent desired throttle angle 82 using observer-based electronicthrottle control (ETC) plate motion dynamics model 80 to generate acurrent estimate 84 of throttle angle position and a future estimate 86of throttle angle position. In a preferred embodiment, future throttleangle position 86 corresponds to a two-PIP ahead event which correspondsto a subsequent fuel injection based on crank angle position.Observer-based model 80 uses a measured throttle angle position 88 toensure stability and compensate for any modeling inaccuracies asdescribed in greater detail below.

The current 84 and future 86 airflow actuator position estimates areprocessed by a throttle body airflow model 90 to generate current 92 andfuture 94 estimates for mass airflow (MAF).

The current 92 and future 94 mass airflow estimates are provided to anobserver-based intake manifold filling model 98 which then provides anestimate of the cylinder air charge 102 for a future fuel injectionevent. Manifold filing model 98 uses a current calculated air charge 102as a feedback element to account for modeling inaccuracies. The currentcalculated air charge 102 is based on the measured or sensed massairflow 96, which is also processed by manifold filling model 98.

FIG. 3 is a block diagram illustrating a general dynamic model for aclosed-loop ETC system for use in determining an estimated futurecylinder air charge according to the present invention. Dynamic model104 captures the dynamics of the system, i.e., the transfer function, sothat a throttle angle output 112 can be predicted or estimated based ona given desired throttle angle input 106. Model 104 combines aclosed-loop throttle controller and plant dynamics model. Input 106 ispreferably the commanded or desired throttle angle while output 112represents the actual throttle angle after the controller has responded.In one preferred embodiment of the present invention, the commanded ordesired throttle angle 106 is generated or sampled at a predeterminedtime interval which is independent of the current engine rotationalspeed (RPM). Model 104 is non-linear and contains a transport delay 108to model the controller delay associated with the commanded throttleangle. The estimated throttle angle position is used to provide afeedback signal which is combined at block 110 with the delayedestimated signal to provide an error or correction term. To model themotor rate-limiting effects and bias spring, a non-linear saturationelement 114 with positive and negative calibratible limits R and F,respectively, is also provided and results in a rate-limited error 116.A second-order linear portion of the model 118 represents plantdynamics. Two calibratable parameters (K and τ) of linear portion 118are functions of both the throttle controller and motor dynamics.Depending upon the particular controller gains selected and motordynamics used, the linear portion 118 of model 104 can be furthersimplified as an integrator with a proportional gain. According to thepresent invention, model 104 is used in a recursive manner to providecurrent and future throttle angle estimates based on an input desiredthrottle angle. To provide an appropriate estimator, model 104 isdiscretized to provide a difference equation and algorithm forestimating the current and future throttle angle positions.

Preferably, model 104 is discretized in the crank-angle domain becauseair charge calculation and intake manifold filling dynamics are executedon a crank-angle domain basis. As such, the sampling interval in theresulting algorithm becomes a function of engine speed. The presence ofboth linear and non-linear components in model 104 requires discretizingthe model in four steps: discretizing the non-linear transport delay,discretizing the non-linear rate-limiter, discretizing the linearfeed-forward frequency-domain plant dynamics model, and developing analgorithm which combines all of the components of the model. Thealgorithm is preferably used recursively to provide one-PIP and two-PIPahead throttle angle estimates. A closed-loop observer structure is thenused for the current throttle angle estimate and one-PIP ahead throttleangle estimate to account for modeling inaccuracies and improvesteady-state stability.

The first step in discretizing the model includes discretizing thetransport delay 108 to support a variable sampling interval. When adesired throttle angle 106 is commanded, controller actuation does nottake place until some later time. In one embodiment, the transport delayis approximately 14 milliseconds. In a discrete domain, the controlleractuation delay is represented by K_(dly) measurement samples.Mathematically this can be represented by:

θ_(DLY)(t)=θ_(DES)(t−t _(DLY))

θ_(DLY)(k)=θ_(DES)(k−k _(DLY))

where: ${k_{DLY} = \frac{t_{DLY}}{T}},$

T is the sample period given by:$T = \frac{2}{\frac{n}{60}\left( N_{CYL} \right)}$

Since k_(DLY) varies:

θ_(DLY)(k)=α[θ_(DES)(k−ceil(k _(DLY)))]+β[θ_(DES)(k−ceil(k _(DLY)−1))]

where:

α=(k _(DLY) −ceil(k _(DLY)−1)), and β=1−α

The use of a ceiling function (ceil (x)) ensures an actual delayedsample is, used.

For a fixed sampling interval, K_(dly) is a fixed quantity. However,since the algorithm is to be executed in the crank-angle domain with avariable sampling interval based on engine speed, K_(dly) is no longer afixed quantity. Therefore, a weighted function of delayed samples of thedesired throttle angle is required to account for a varying samplinginterval. According to the present invention, ranges of both thesampling interval and K_(dly) must be identified so that a minimumhistory of delayed desired throttle angle samples is used in thealgorithm. For an eight-cylinder engine with engine speeds rangingbetween 650 and 7000 RPM, ranges of the sampling interval and K_(dly)are:

2.1 ms≦T≦23.1 ms

and

6.667≧k _(DLY)≧0.6087

With this range of K_(dly), a weighted function of delayed samples ofthe desired throttle angle that ranges from the current desired throttleangle to a desired throttle angle seven samples old should be used.Therefore, a seven-sample history of the desired throttle angle is usedto integrate the transport delay (14 ms in this example) into thealgorithm. Of course, the number of samples maintained in the samplehistory will vary depending upon the particular application, but may bedetermined according to the process described above.

A second step in discretizing model 104 is to discretize the non-linearrate limiter used to model the throttle motor rate limiting effects andbias spring. Using a piecewise linear relationship after calculating theerror term e_(K), which is the error between the desired and actualthrottle position, the rate limiter can be discretized as follows:

 e _(K)=θ_(DLY) _(K) −{circumflex over (θ)}_(EST) _(K)

where: $e_{K} = {{f_{ratelimit}\left( e_{k} \right)}\begin{Bmatrix}{R,{{{if}\quad e_{k}} > R}} \\{F,{{{if}\quad e_{K}} < F}} \\{e_{K},{otherwise}}\end{Bmatrix}}$

The above equations are also adaptable for use when an observer is addedto model 104 as illustrated and described with reference to FIGS. 4 and5. The representations for the non-linear rate-limiter for the currentthrottle angle and one-PIP ahead estimates are:

e _(K)=ƒ_(ratelimit)(e _(K))+L(θ_(ACT) _(K−1) −{circumflex over(θ)}_(EST) _(K−1) )

e _(K+1)=ƒ_(ratelimit)(e _(K+1))+L(θ_(ACT) _(K) −θ_(EST) _(K) )

Preferably, the error is calculated after the current throttle angle isestimated.

The third step in discretizing the actuator model 104 is to discretizethe feed-forward throttle control dynamics model 118. Because the modelis linear, this can be accomplished using a Z-transform approach byapplying any of a number of methods such as ZOH, BilinearTransformation, Backward Euler, and the like. The BilinearTransformation method, also known as Tustin's method, uses a trapezoidalrule for numerical integration approximation and provides a moreaccurate mapping of the plant dynamics in the discrete domain for anelectronic throttle airflow actuator. Preferably, the BilinearTransformation method is used because it is less sensitive to varyingsampling intervals compared to other methods such as ZOH or BackwardEuler methods which may introduce oscillatory and/or unstable behaviorwhen large sampling intervals exist. The discrete domain transferfunction G(z) for the frequency-domain plant dynamics model G(s) may berepresented as follows:${G(s)} = \frac{K}{s\left( {{\tau \quad s} + 1} \right)}$$\begin{matrix}{{G(z)} = {{G(s)}_{s = \frac{2{({z - 1})}}{T{({z - 1})}}}}} \\{= \frac{{KT}^{2}\left( {z^{2} + {2z} + 1} \right)}{{z^{2}\left( {{4\tau} + {2T}} \right)} - {8\tau \quad z} + \left( {{4\tau} - {2T}} \right)}} \\{= \frac{{\hat{\theta}}_{EST}(z)}{e(z)}}\end{matrix}$

The discrete domain transfer function is then used to derive acorresponding difference equation for F≦e_(K)≦R as follows:${\hat{\theta}}_{{EST}_{K}} = {\frac{1}{\left( {1 + {b_{0}/a_{0}}} \right)}{\left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)\theta_{{DLY}_{K}}} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K - 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{k - 2}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 2}}}} \right\rbrack}}$

where:

b ₀ =KT ² , b ₁=2KT ² , b ₂ =b ₀ , a ₀=(4τ+2T), a ₁=−8τ, Λa ₂=(4τ−2T)

For e_(K)>R, the difference equation is:${\hat{\theta}}_{{EST}_{K}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)R} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K - 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 2}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 2}}}} \right\rbrack$

and for e_(K)<F, the difference equation is:${\hat{\theta}}_{{EST}_{K}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)F} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K - 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 2}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 2}}}} \right\rbrack$

The discretized components are then combined to form an algorithm forestimating the current throttle angle based on any desired throttleangle input.

FIGS. 4 and 5 provide block diagrams illustrating a recursive airflowactuator position model with a closed-loop observer structure usingsensed throttle plate position with a proportional gain to account formodeling inaccuracies and to ensure correct prediction of throttle plateposition. FIG. 4 provides a simplified representation of the recursiveairflow actuator position model for estimating current throttle valveposition, and first and second future throttle valve positions accordingto the present invention. Recursive model 130 includes a desiredthrottle valve position 132 as an input. As described above, the desiredor commanded throttle valve angle is sampled or provided on apredetermined (fixed) sampling interval. Relative to the crank-angledomain, the desired throttle valve position is considered to be constantfor the current, and two future events. The desired throttle valveposition 132 is used to provide an estimated current position 134 withan associated error 135 based on the ETC plate motion dynamic model 136.Outputs 134 and 135 are provided recursively to blocks 138 and 140. Thecurrent estimated throttle position 134 is used to predict a firstfuture estimated throttle position 139 using block 138. An associatedfuture error value 141 is also provided. As such, block 138 provides anestimated airflow actuator position for a first future engine event(fuel injection or intake event in this example) based on a currentestimated position. The first future estimated position value isprovided as an input to block 140 to generate a second future estimatedposition value 142 and associated error 143. In one preferredembodiment, the second future estimated value 142 is determined for atwo-PIP ahead throttle angle estimate. A measured value 144corresponding to the current throttle valve position provides feedbackto model 130 to account for modeling inaccuracies and ensure correctprediction of future throttle plate positions as best illustrated inFIG. 5.

A more detailed representation of a recursive observer-based model forestimating future airflow actuator position according to the presentinvention is illustrated in FIG. 5. A closed-loop observer structure 154is added to the dynamic throttle position model 104′ using a measured orsensed throttle position 158 provided by the closed-loop ETC systemcontroller 156 as a feedback signal. The measured signal is time shiftedas represented by block 160 and compared with the current estimate 134,which is time shifted as represented by block 162, to generate an erroror difference signal. A proportional gain 164 is applied and used toadjust or modify model 104 based on an error or correction factor 135.

The desired throttle valve position for a first future event 170 isprovided as an input to another instance of model 104′ to predict afirst future estimated position 139. As illustrated by the broken line,the future desired value may be assumed to be equal to the currentdesired value 150 and the second future desired value 172 for mostapplications. Input 170 is processed by model 104 to determine a firstfuture estimated value for throttle position 139. The current estimatedvalue 134 is used by an observer structure 174 to account for modelingerrors and provide a feedback signal based on the current actualthrottle valve position at block 176. A proportional gain 178 is appliedand the error 141 is added to the model 104′.

Similarly, a desired throttle valve position for a second future event172 is processed by model 104 to determine an estimated throttle angleposition for a second future event 142 with an associated feedback error143.

The closed-loop observer structures 154 and 174 of FIG. 5 are used onlyfor the current and first future estimate for throttle angle positionand not for the second future estimate. To use a closed-loop observerstructure for the second future event would require a sensed throttleposition for the second future event which is not available. However,the feedback provided for the current and first future estimates isdynamically coupled with the second future estimate. As such, the secondfuture estimate will be improved as a result of integrating (combining)a closed-loop observer with the current and first future estimates.

In one embodiment, the present invention executes a recursive algorithmassuming that the desired throttle angle is the same for the current,first future event, and second future event. This assumption is valid solong as the intake events occur at a faster rate than the update rate ofthe desired throttle angle. In situations where this assumption is notvalid, the closed-loop observer structure illustrated in FIGS. 4 and 5will make appropriate corrections. One implementation for a recursivealgorithm according to the present invention is as follows:${\hat{\theta}}_{{EST}_{K}} = {\frac{1}{\left( {1 + {b_{0}/a_{0}}} \right)}{\left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)\theta_{{DLY}_{K}}} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K - 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 2}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 2}}}} \right\rbrack}}$

For e_(K)>R,${\hat{\theta}}_{{EST}_{K}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)R} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K - 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 2}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 2}}}} \right\rbrack$

and for e_(K)<F,${\hat{\theta}}_{{EST}_{K}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)F} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K - 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 2}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 2}}}} \right\rbrack$

where

e _(K)=ƒ_(ratelimit)(θ_(DLY) _(K) −{circumflex over (θ)}_(EST) _(K))+L(θ_(ACT) _(K−1) −{circumflex over (θ)}_(EST) _(K−1) )

For the first and second future position values assuming constantdesired values and constant delay values (equal to the current desiredvalue and current delay value), the first future position value is givenby:${\hat{\theta}}_{{EST}_{K + 1}} = {\frac{1}{\left( {1 + {b_{0}/a_{0}}} \right)}\left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)\theta_{{DLY}_{K + 1}}} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 1}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}}} \right\rbrack}$

For e_(K+1)>R,${\hat{\theta}}_{{EST}_{K + 1}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)R} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 1}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K}}} - {1\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}}} \right\rbrack$

and for e_(K+1)<F,${\hat{\theta}}_{{EST}_{K + 1}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)F} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K - 1}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K - 1}}}} \right\rbrack$

where

e _(K−1)=ƒ_(ratelimit)(θ_(DLY) _(K−1) −{circumflex over (θ)}_(EST)_(K+1) )+L(θ_(ACT) _(K) −{circumflex over (θ)}_(EST) _(K) )

For the second future position value of the throttle valve,${\hat{\theta}}_{{EST}_{K + 2}} = {\frac{1}{\left( {1 + {b_{0}/a_{0}}} \right)}{\left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)\theta_{{DLY}_{K + 2}}} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K + 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K + 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K}}}} \right\rbrack}}$

For e_(K+2)>R,${\hat{\theta}}_{{EST}_{K + 2}} = \left\lbrack {{\left( \frac{b_{0}}{a_{0}} \right)R} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K + 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K + 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K}}}} \right\rbrack$

and for e_(K+2)<F,${\hat{\theta}}_{{EST}_{K + 2}} = \left\{ {{\left( \frac{b_{0}}{a_{0}} \right)F} + {\left( \frac{b_{1}}{a_{0}} \right)e_{K + 1}} + {\left( \frac{b_{2}}{a_{0}} \right)e_{K}} - {\left( \frac{a_{1}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K + 1}}} - {\left( \frac{a_{2}}{a_{0}} \right){\hat{\theta}}_{{EST}_{K}}}} \right\}$

where:

e _(k+2)=ƒ_(ratelimit)(θ_(DLY) _(K+2) −{circumflex over (θ)}_(EST)_(K+2) )

FIG. 6 is a block diagram illustrating estimation of intake airflow andmanifold filling effects based on estimated throttle plateangleaccording to one embodiment of the present invention. Once thecurrent and future throttle plate angles are estimated as describedabove, the present invention utilizes an airflow model based on theactuator positions in conjunction with an intake manifold filling modelto predict the in-cylinder air charge for a future injection event.Depending upon the particular engine technology and airflow actuator,the intake manifold filling model may be modified or eliminated. Forexample, for applications using variable valve timing with athrottle-less engine, the actuator position model and/or airflow modelmay incorporate the manifold filling effects. Likewise, any two or moreof the models may be combined with appropriate modifications withoutdeparting from the spirit or scope of the present invention.

In one preferred embodiment, an electronically controlled throttle valveis used alone or in conjunction with controllable valve timing toprovide airflow control. As such, the current and future throttle plateangles are used in an adaptive throttle body airflow model to providecorresponding current and future estimates of mass airflow into theintake manifold 190. To account for modeling inaccuracies, a futuredelta MAF is generated by taking the difference of the current andfuture estimated mass airflows. The future delta mass airflow is thenused as a feed-forward term with the sensed mass airflow.

An unadapted throttle body mass airflow model may be based on thefollowing adiabatic orifice flow equations:${{MAF} = {C_{D}A_{th}p_{atm}\sqrt{{\frac{2\kappa}{\kappa - 1}\left\lbrack {\left( \frac{\rho_{man}}{\rho_{atm}} \right)^{\frac{2}{\kappa}} - \left( \frac{\rho_{man}}{\rho_{atm}} \right)^{\frac{\kappa + 1}{\kappa}}} \right\rbrack} \cdot \frac{M}{\overset{\_}{R}T}}}}\quad$$\quad {{{{for}{\quad \quad}\frac{\rho_{man}}{\rho_{atm}}} > {\left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa}{\kappa - 1}}\quad {and}}}\quad \quad {{MAF} = {C_{D}A_{th}\rho_{atm}\sqrt{\kappa \left\lbrack \left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa + 1}{\kappa - 1}} \right\rbrack}\frac{M}{\overset{\_}{R}T}\quad {for}}}\quad {\frac{\rho_{man}}{\rho_{atm}} \leq \left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa}{\kappa - 1}}}}$

where C_(D) represents discharge coefficient (determined empirically),A_(th) represents effective throttle flow area, _(atm) representsatmospheric pressure, _(man) represents downstream intake manifoldpressure, represents ratio of specific heats, M represents molecularweight of gas, T represents upstream temperature of air charge,{overscore (R)} represents ideal gas constant, and MAF represents massairflow entering the throttle body.

These equations are highly non-linear and are preferably regressed fromempirical mapping data based on a particular size of throttle body orthrottle plate. Preferably, the mass airflow model includes inputscorresponding to current barometric (atmospheric) pressure (BP), aircharge temperature (ACT), intake manifold pressure (MAP), and throttleangle. For the first and second future mass airflow estimates, theintake manifold pressure and air charge temperature are assumed to bethe same as the current values. In one preferred embodiment, intakemanifold pressure corresponds to a desired manifold pressure commandedby the electronic throttle controller rather than an actual measured orsensed value. These assumptions are valid for air charge estimationpurposes with any inaccuracies removed because only a delta mass airflowis to be generated.

With reference to FIG. 6, air is inducted through intake 180 and passesby mass airflow sensor 182 before entering throttle body 184. Intake airis modulated by position of throttle plate 188 with a measured or sensedposition determined by throttle plate position sensor 186. Air passingthrough throttle body 184 enters intake manifold 190 where it isdistributed to the various cylinders 204. Fuel injector 192 injects anappropriate quantity of fuel which is entrained as the air passes intocylinder 202 upon the opening of intake valve (or valves) 194. Intakevalve 194 is closed as piston 206 rises during the compression stroke.An appropriate signal is provided to spark plug 200 for combustion tooccur within chamber 202. Exhaust valve (or valves) 196 is then openedand the combusted gases pass into exhaust manifold 198. According to thepresent invention, air/fuel ratio control is improved by estimatingposition of throttle plate 188 to determine airflow into intake manifold190. Any modeling errors are canceled out and are compensated for usingfeedback provided by throttle position sensor 186 and by using themodels to estimate a delta mass airflow. A manifold filling model isused to provide an estimate of the air charge entering combustionchamber 202 from manifold 190 during the subsequent intake event whenintake valve 194 is open and an air/fuel mixture is provided to chamber202.

A block diagram illustrating one embodiment for a control system withfeed-forward observer-based controls for future in-cylinder air chargeestimation according to the present invention is shown in FIG. 7. TheETC plate motion dynamic model 220 is used in conjunction with anairflow model 230 and manifold filling model 252 to estimate or predictthe in-cylinder air charge 256 for a future engine event, such as anintake or fuel injection event. A desired throttle angle 226 is used togenerate a current estimate 222 and a future estimate 224 of thethrottle plate position. Feedback 228 is provided by measuring orsensing position of the throttle valve and adjusting model 220according.

Throttle flow model 230 uses the current and estimated throttle valvepositions 222 and 224, respectively, along with air charge temperature236, barometric pressure 238, and desired intake manifold pressure 240to generate estimates for the current mass airflow (MAF) 232 and afuture mass airflow 234. The difference between the current and futureestimated mass airflow values is determined at 242 to generate a futuredelta mass airflow estimate 244. This result provides a feed-forwarddelta mass airflow prediction to be added to the current mass airflowreading 248 at block 246. Block 250 integrates the result to produce acurrent intake air charge which is then provided directly to an intakemanifold filling model 252 for a final in-cylinder air charge estimate256 for a future engine event. In one preferred embodiment of thepresent invention, the engine event corresponds to a fuel injectionevent with the estimated air charge 256 being used by the fuel controlto schedule an appropriate fuel quantity to achieve the desired air/fuelratio.

By using the same throttle flow model 230 to process the current 222 andfuture 224 estimated throttle valve positions, and taking the differenceof the predicted mass airflow at block 242, most modeling errors arecanceled whether induced by the ETC plate motion dynamic model 220 orthe throttle flow model 230.

Intake manifold filling model 252 is used to account for filling effectsduring transients which would otherwise lead to undesirable air/fuelratio excursions. During transients, the difference between the sensedmass airflow and the in-cylinder airflow is equal to the rate of changeof the air mass in the intake manifold. Under steady-state conditions,the sensed mass airflow is equal to the in-cylinder airflow. Treatingthe engine as a volumetric pump, using the ideal gas law, and applyingconservation of mass to the intake manifold, the manifold fillingdynamics can be represented as:${\overset{.}{M}}_{MAN} = {\left. {{MAF} - {\overset{.}{M}}_{CYL}}\Rightarrow\frac{{\overset{.}{P}}_{{MANV}_{MAN}}}{{RT}_{MAN}} \right. = {{MAF} - \frac{\eta_{v}P_{MAN}V_{d}n}{120{RT}_{MAN}}}}$

in the frequency domain, this is represented as:${\frac{M_{CYL}(s)}{{MAF}(s)} = \frac{1}{{\tau_{MAN}s} + 1}},\quad {{{where}\quad \tau_{MAN}} = {120\frac{V_{MAN}}{\eta_{V}V_{D}n}}}$

Discretizing the frequency domain transfer function above into the crankangle domain leads to:

M _(CYL) _(K) =(1−air _(—) fk)M _(CYL) _(K−1) +MAF _(K)(air _(—) fk)

where air_fk can be approximated as:$\frac{\eta_{V}V_{D}}{N_{CYL}V_{MAN}}$

and where MAF represents mass airflow past the throttle body, M_(CYL)represents mass airflow into the cylinder, M_(MAN) represents rate ofchange of mass airflow in the intake manifold, P_(MAN) represents intakemanifold pressure, V_(MAN) represents intake manifold displacementvolume, T_(MAN) represents intake manifold temperature, R represents gasconstant, V_(D) represents cylinder displacement volume, _(v) representsvolumetric efficiency, n represents engine speed (RPM), and T_(MAN)represents a manifold filling time constant.

Another embodiment of a feed-forward observer-based control according tothe present invention is illustrated in FIG. 8. Similar to theembodiment illustrated in FIG. 7, the embodiment of FIG. 8 includes anETC plate motion model 220 which generates estimates for a current 222and future 224 throttle plate position corresponding to a desired 226throttle plate position. Feedback 228 is provided based on a sensed ormeasured throttle plate position. Current estimate 222 is provided tothrottle flow model 230 along with future estimate 224. Air chargetemperature (ACT) 236, barometric pressure (BP) 238, and desired,estimated, or measured manifold absolute pressure (ETC_DES_MAP) 240 areused by throttle flow model 230 in generating a current MAF estimate 232and a future MAF estimate 234.

The current and future MAF estimates are integrated as indicated at 272and 274 prior to being processed by manifold filling model 252. Based onthe current MAF estimate, manifold filling model 252 generates acorresponding air charge estimate which is combined with an air chargeestimate based on the future estimated MAF at block 276. A differentialfuture air charge estimate 278 is produced and used as a feed-forwardterm at 282 where it is combined with air charge 280 calculated based onMAF sensor signal 248 after passing through integrator 270 and beingprocessed by manifold filling model. 252. The resulting air chargeestimate for in-cylinder air charge at a future engine event,represented generally by reference numeral 290, may then be used toschedule an appropriate amount of fuel to more precisely controlair/fuel ratio.

Similar to the embodiment illustrated in FIG. 7, the embodiment of FIG.8 eliminates modeling errors induced by ETC plate motion model 220,throttle flow model 230, and manifold filling model 252 through thedifference operation represented by block 276. As will be appreciated bythose skilled in the art, the differencing operation effectively cancelsany modeling error since the error will be present for both the currentand future estimated values. The resulting delta air charge 278 may thenbe used as a feed-forward term to improve the responsiveness of thecontrol system.

FIG. 9 provides a flowchart illustrating operation of two embodimentsfor a system and method for air charge estimation according to thepresent invention. The diagram of FIG. 9 represents control logic of oneembodiment of a system or method according to the present invention. Aswill be appreciated by one of ordinary skill in the art, the diagrams ofthe various Figures may represent any one or more of a number of knownprocessing strategies such as event-driven, interrupt-driven,multi-tasking, muti-threading, and the like. As described above, thepresent invention preferably utilizes both an event-driven strategytriggered by a particular event, i.e., intake or fuel injectioncorresponding to a particular crank angle, in combination withtime-domain, fixed-interval interrupt processing, such as used forcalculation of a desired throttle angle, for example. Thus, varioussteps or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the objects,features, and advantages of the present invention, but is provided forease of illustration and description. Although not explicitlyillustrated in FIG. 9, one of ordinary skill in the art will recognizethat one or more of the illustrated steps or functions may be repeatedlyperformed depending upon the particular function and the processingstrategy being used.

Preferably, the control logic illustrated in FIG. 9 is primarilyimplemented in software which is executed by a microprocessor-basedengine controller. Of course, the control logic may be implemented insoftware, hardware, or a combination of software and hardware dependingupon the particular application. When implemented in software, thecontrol logic is preferably provided in a computer-readable storagemedium and in stored data representing instructions executed by acomputer to control the engine. The computer-readable storage medium ormedia may be any of a number of known physical devices which utilizeelectric, magnetic, and/or optical devices to temporarily orpersistently store executable instructions and associated calibrationinformation, operating variables, parameters, and the like.

In the embodiment of FIG. 9, block 300 represents estimating a currentairflow actuator position based on a current desired position. First andsecond future positions are estimated as represented by block 302 usinga recursive actuator position model as described in detail above. In oneembodiment, the first and second future positions are estimated assumingthe desired position remains constant. A corresponding measured positionis used along with at least one of the first and second future positionsto adjust the model as indicated by block 304. Current and futureestimates for mass airflow based on estimated current and futureactuator positions, respectively, are determined as represented by block306.

Blocks 308-314 represent a first embodiment using feed-forwardobserver-based control according to the present invention. Blocks316-322 represent an alternative embodiment for feed-forwardobserver-based control according to the present invention. Asrepresented by block 308, the current and future airflows are comparedto generate a future delta mass airflow estimate. The future delta massairflow estimate eliminates most modeling errors which may be present ineither the actuator position model or the airflow model used todetermine the current and future mass airflow values. The delta massairflow estimate is used as a feed-forward term and combined with ameasured mass airflow value (MAF) as represented by block 310. Thisprovides an adjusted MAF estimate which is then integrated as indicatedat block 312. The integrated adjusted estimate is processed using amanifold filling model as represented by block 314 to determine thefuture in-cylinder air charge.

One alternative embodiment is illustrated with reference to blocks316-322. Rather than combining the current and future estimated airflowsat block 308, block 316 integrates the current and future MAF estimatesand processes each separately using the manifold filling model asrepresented by block 318. The resulting current and future air chargeestimates provided by the manifold filling model are used to generate adelta air charge. The delta air charge cancels most modeling errorsinduced by the airflow actuator model, the airflow model, and/or themanifold filling model. The delta air charge value is used as afeed-forward term as represented by block 320. The feed-forward term iscombined with an air charge determined using the manifold filling modelwith a measured MAF.

As such, the present invention provides various systems and methods forfeed-forward observer-based controls to estimate the in-cylinder aircharge for a future engine event, such as a cylinder intake and/orinjection event. According to the present invention, modeling errors areeliminated without the need for adaptive controls which may requireincreased development, validation, and testing resources in comparison.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

What is claimed is:
 1. A method for improving air/fuel ratio control inan internal combustion engine having an electronically controlledthrottle valve to control intake airflow, the method comprising:estimating a current throttle valve position and a future throttle valveposition based on a desired throttle valve position using a throttlevalve motion model with feedback from a sensed position of the throttlevalve; determining an estimated current mass airflow and future massairflow based on the estimated current and future throttle valvepositions, respectively; combining the estimated current and future massairflows to generate a future delta mass airflow estimate; combining thefuture delta mass airflow estimate with a value representing a measuredmass airflow to generate an adjusted future mass airflow estimate; anddetermining a future in-cylinder air charge based on the adjusted futuremass airflow estimate.
 2. The method of claim 1 wherein the step ofestimating a current throttle valve position and a future throttle valveposition comprises: estimating a current throttle valve position basedon a current desired throttle valve command; using the estimated currentthrottle valve position to generate a first future throttle valveposition corresponding to a first subsequent fuel injection event; andgenerating a second future throttle valve position estimate based on thefirst future throttle valve position estimate.
 3. The method of claim 1wherein the step of determining an estimated current mass airflow and anestimated future mass airflow comprises: determining an estimatedcurrent mass airflow based on the estimated current throttle valveposition; and determining an estimated future mass airflow based on thesecond future throttle valve position estimate.
 4. The method of claim 3wherein the steps of determining estimated current and future massairflow comprise estimating current and future mass airflow based oncurrent air charge temperature, barometric pressure, and manifoldpressure.
 5. The method of claim 1 wherein the step of determining afuture in-cylinder air charge comprises processing the adjusted futuremass airflow estimate to account for manifold filling effects.
 6. Themethod of claim 5 further comprising integrating the adjusted futuremass airflow estimate prior to processing to account for manifoldfilling effects.
 7. A method for improving air/fuel ratio control in aninternal combustion engine having an electronically controlled throttlevalve, the method comprising: estimating current and future positions ofthe throttle valve using a throttle valve position model; estimatingcurrent and future mass airflow corresponding to the current and futureposition estimates of the throttle valve, respectively; determining acurrent estimated in-cylinder air charge and a future estimatedin-cylinder air charge based on the current and future mass airflowestimates, respectively; comparing the current and future estimatedin-cylinder air charge to generate a future delta air charge; anddetermining a final future estimated air charge based on the futuredelta air charge.
 8. The method of claim 7 wherein the step ofdetermining a final future estimated air charge comprises: determining ameasured cylinder air charge based on a measured mass airflow using amanifold filling model; and adding the measured cylinder air charge tothe future delta air charge to determine the final future estimated aircharge.
 9. The method of claim 7 wherein the step of estimating acurrent throttle valve position and a future throttle valve positioncomprises: estimating a current throttle valve position based on acurrent desired throttle valve command; using the estimated currentthrottle valve position to generate a first future throttle valveposition corresponding to a first subsequent fuel injection event; andgenerating a second future throttle valve position estimate based on thefirst future throttle valve position estimate.
 10. The method of claim 7further comprising integrating the current mass airflow estimate and thefuture mass airflow estimate.
 11. The method of claim 7 wherein the stepof estimating current and future mass airflow comprising estimatingcurrent and future mass airflow based on current air charge temperature,barometric pressure, and manifold pressure.
 12. A system for controllingan internal combustion engine to improve air/fuel ratio control byestimating future in-cylinder air charge, the system comprising: anairflow actuator for modulating intake air into the internal combustionengine; a mass airflow sensor for measuring mass airflow through anintake passage into the internal combustion engine; a position sensorfor measuring position of the airflow actuator; and a controller incommunication with the airflow actuator, the mass airflow sensor, andthe position sensor, the controller estimating current and futurepositions of the airflow actuator using an actuator model, adjusting theactuator model based on at least one of the current and future positionestimates and a measured position based on a signal provided by theposition sensor, estimating current and future mass airflow valuescorresponding to the current and future position estimates, andcombining the difference between the current and future airflow valueswith a sensed airflow value based on a signal provided by the massairflow sensor to estimate the future in-cylinder air charge.
 13. Thesystem of claim 12 wherein the controller combines the current andfuture airflow values by estimating a current and future in-cylinder aircharge based on the current and future mass airflow estimates,respectively, compares the current and future in-cylinder air charge togenerate a future delta air charge estimate, and combines the futuredelta air charge estimate with an air charge based on a measured massairflow to determine a final estimate for the future in-cylinder aircharge.
 14. The system of claim 12 wherein the controller combines thecurrent and future airflow values to generate a future estimated deltamass airflow, and processes the future estimated delta mass airflow todetermine the future in-cylinder air charge.
 15. The system of claim 14wherein the controller processes the future estimated delta mass airflowby combining a measured mass airflow value with the future estimateddelta mass airflow.